1. Field of the Invention
The present invention relates to a pressure control apparatus which makes it possible to control the pressure of a pressure fluid to be supplied, for example, to a fluid pressure-operated apparatus such as a cylinder.
2. Description of the Related Art
A pressure control valve concerning a conventional technique is shown in FIG. 6 (see Japanese Laid-Open Patent Publication No. 5-158552). The pressure control valve 1 includes a main valve 3 for controlling the pressure of air derived from a compressed air supply source 2 to supply the air to a fluid pressure-operated apparatus such as a cylinder (not shown) connected to the secondary side, an air-supplying solenoid-operated valve 4 for controlling the pressure to be supplied to a pilot chamber of the main valve 3, and an air-discharging solenoid-operated valve 5 for controlling the pressure of the air discharged from the pilot chamber to the outside.
The air-supplying solenoid-operated valve 4 is composed of a normally closed type solenoid-operated valve which maintains the shut off state when the electric power application is stopped. On the other hand, the air-discharging solenoid-operated valve 5 is composed of a normally open type solenoid-operated valve which maintains the open state when the electric power application is stopped.
Further, the pressure control valve 1 is provided with a pressure sensor 6 for detecting the pressure on the output side of the main valve 3, and a control circuit 7 for making control to open and close the air-supplying solenoid-operated valve 4 and the air-discharging solenoid-operated valve 5 respectively on the basis of a detection signal derived from the pressure sensor 6.
The pressure control valve 1 makes control to open and close the air-supplying solenoid-operated valve 4 and the air-discharging solenoid-operated valve 5 at a high speed respectively in accordance with a duty ratio of a pulse signal outputted from the control circuit 7 on the basis of to a difference (deviation) between the pressure on the output side and the preset pressure. Accordingly, the air, which is controlled to have a desired pressure, is outputted.
As shown in FIG. 7, Japanese Laid-Open Patent Publication No. 5-158552 described above discloses the following arrangement as another embodiment for substituting the P portion shown in FIG. 6. That is, an air-supplying solenoid-operated valve 4 and an air-discharging solenoid-operated valve 8, each of which is composed of a normally closed type solenoid-operated valve, are provided. Further, another solenoid-operated valve 9, which functions to release or maintain the secondary pressure of the main valve 3 when the power source is cut off, is arranged between the pilot chamber and the air-supplying solenoid-operated valve 4 and the air-discharging solenoid-operated valve 8.
However, the following inconvenience arises in the case of the pressure control valve 1 concerning the conventional technique in which the air-discharging solenoid-operated valve 5 as the normally open type solenoid-operated valve as shown in FIG. 6 is adopted. That is, in order to maintain a constant pilot pressure in the state of normal operation, it is necessary that the air-discharging solenoid-operated valve 5 is always in the state of electric power application. For this reason, the unillustrated solenoid coil of the air-discharging solenoid-operated valve 5 generates the heat. Further, the ambient atmosphere has a high temperature due to the heat generated by the solenoid coil. As a result, another inconvenience also arises such that any temperature drift occurs in the pressure sensor 6.
In view of the above, when the air-discharging solenoid-operated valve 8, which is the normally closed type solenoid-operated valve, is adopted in place of the normally open type solenoid-operated valve, it is necessary to provide the another solenoid-operated valve 9 to release or maintain the secondary pressure of the main valve 3 when the power source is cut off as shown in FIG. 7. It is required that the another solenoid-operated valve 9 is always in the state of electric power application in the state of normal operation. Therefore, an inconvenience arises such that the solenoid coil of the another solenoid-operated valve 9 generates the heat.
A general object of the present invention is to provide a pressure control apparatus in which a normally closed type air-discharging solenoid-operated valve is used to allow the air-discharging solenoid-operated valve to be in the ON state only when the power source is cut off, making it possible to avoid the heat generation of the air-discharging solenoid-operated valve in the state of normal operation.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.